Several methods exist to monitor a target substance in a biological system, including monitoring activities inside a cell and intracellular processes. These methods utilize various reporter molecules and genes and other molecular tags and include, for example, the formation of fusion proteins with coding sequences for chloramphenicol acetyl transferase (CAT), gluceronidase (GUC), beta-galactosidase (bGAL) and luciferases (LUC). Many of these reporters and tags may be used as indicators of processes that would be hard to detect otherwise. Silhavy, T. J. & Beckwith, J. R. Microbiol. Rev. (49), 398 (1985); Gould, S. J. & Subramani, S. Anal. Biochem. (175), 5 (1988); and Stewart, G. S. A. B. & Williams, P. J. Gen. Microbiol. (138), 1289-(1992).
Many uses of these reporter genes have been described extensively in the prior art. There use, however, has been limited because they require extra manipulations. For example, the fixing of cell preparations or the addition of exogenous substrates or cofactors makes it difficult to incorporate the use of these reporters in highly automated assays. In contrast, the use of another reporter gene, the green fluorescent protein (GFP), is not limited by these restrictions but suffers from other disadvantages, such as background noise and the need for complex and expensive equipment for detection. Cormack, B. P., et al. Gene, 173(1):33–38 (1996); Kroes, S. J., et al. Eur. J. Biochem. 240(2), 342–351 (1996).
Furthermore, until recently, detection in automated assays was based on enzymatic or fluorimetric methods for reasons of sensitivity. As detection at the microscopic level has developed, however, it has become technically possible to use markers without amplification of the marker itself, or signal amplification. Thus, the development of novel reporter genes or markers that have none of the above limitations and disadvantages are desired.